In situ cosmogenic radiocarbon production and 2‐D ice flow line modeling for an Antarctic blue ice area

Abstract

Radiocarbon measurements at ice margin sites and blue ice areas can potentially be used for ice dating, ablation rate estimates and paleoclimatic reconstructions. Part of the measured signal comes from in situ cosmogenic 14C production in ice, and this component must be well understood before useful information can be extracted from 14C data. We combine cosmic ray scaling and production estimates with a two‐dimensional ice flow line model to study cosmogenic 14C production at Taylor Glacier, Antarctica. We find (1) that 14C production through thermal neutron capture by nitrogen in air bubbles is negligible; (2) that including ice flow patterns caused by basal topography can lead to a surface 14C activity that differs by up to 25% from the activity calculated using an ablation‐only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model two‐dimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future 14C and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of paleoatmospheric 14C content of methane.

Measurements of 14C in ancient ice from Taylor Glacier, Antarctica constrain in situ cosmogenic 14CH4 and 14CO production rates

“…The ice drilling site for this study (77˚45.699'S, 161˚43.179'E, 527 m asl; Figure 2) is located along the center flowline of the glacier 14.0 km from the glacier terminus. The ice flow velocity at this site is ≈ 10 m / year, with an ablation rate of 0.196 ± 0.020 m / year (Bliss et al, 2011;Buizert et al, 2012).…”

“…Production via fast muon interactions with 16 O in quartz has been demonstrated in the laboratory (Heisinger et al, 2002a) but not confirmed in a natural setting. Production of 14 C by thermal neutron capture from 14 N in air trapped in the ice has been shown to be insignificant (Buizert et al, 2012). Production rates decrease exponentially with depth for the neutron mechanism:…”

“…Glacier (Buizert et al, 2012). In this case, the cumulative 14 C from fast muon production (whose depth dependence is approximated by a 3-term exponential) is given by:…”

Radiometric 81 Kr dating identifies 120,000-year-old ice at Taylor Glacier, Antarctica

“…A promising new technique uses the accumulated recoil 234 U in the ice matrix from 238 U decay in dust grains as an age marker (14); currently the method still has a fairly large age uncertainty (16-300 ka). Flow modeling can provide useful constraints on the age of BIAs (15)(16)(17)(18), but large errors are introduced by the limited availability of field data and necessary assumptions about past flow and ice thickness. Finally, the continued degassing of 40 Ar from the solid Earth allows dating of the ancient air trapped in the ice by evaluating the δ 40 Ar/ 38 Ar and δ 40 Ar/ 36 Ar stable isotope ratios (19), but the current uncertainty in this dating method is also large (180 ka or 11% relative age, whichever is greater).…”

GGR Biennial Critical Review: Analytical Developments Since 2010

“…Buizert et al . () have combined cosmic ray scaling and production estimates with a two‐dimensional ice flow model to study in situ ‐produced 14 C at Taylor Glacier, Antarctica. They found that the thermal neutron production of 14 C in gas bubbles of glacial ice is negligible, but that solar modulation and bedrock topography can significantly affect in situ 14 C production.…”